You can enter the "Friends" section by selecting the two-people icon in the lower right corner of your main screen.
If your game account is linked to a Facebook profile, you will find your list of friends from that social network with whom you can play. Next to each person you will see the options to chat and start the game.
In the upper right corner, next to the search bar, you will have the option to invite friends. By doing so, the game will allow you to send invitations through the different applications that you have installed on your device.